The long-term objective of this project is to provide a complete kinetic and thermodynamic description of the sodium/potassium pump, or (Na+K)ATPase. The sodium/potassium pump is present in the membrane of nearly all animal cells, where it is responsible for maintenance of the transmembrane Na and K gradients, volume and pH regulation, fluid secretion, sensory transduction, secondary transport of nutrients and other ions, etc. The electrogenic nature of the pump affects sensory adaptation, pacemaker generation, and synaptic modulation. The action of digitalis drugs on the heart is almost certainly through the sodium pump. The sodium pump in man consumes an appreciable fraction of all ATP made in the body. The specific aims of this proposal are to continue the analysis of kinetic and thermodynamic relationships between the various modes of operation of the sodium pump, with special attention to the effect of cell membrane potential on the currents and/or fluxes generated by the pump, and the kinetic parameters (individual rate constants; Km's; Ki's; Hill coefficients) governing them. Modes of operation to be investigated include, besides forward and backward Na/K exchange, such behaviors as K/K exchange, electrogenic and electroneutral Na/Na exchange, and voltage-operated movement through reversal potential. Results of these experiments will be tested by rigorous least-squares analysis against specific theoretical (but always "minimal") models of the pump. Voltage-sensitive rate constants will be interpreted in terms of Eyring-barrier models for individual steps in the Albers-Post scheme for the sodium/potassium pump. The cell chosen for this work is a giant nerve fiber of the squid which, because of its very large size, allows experimental access and ionic and biochemical control by means of the internal dialysis technique, as well as electrical control by means a very stable voltage-clamp circuit. Pump-mediated fluxes and current will be defined as cardiotonic steroid-induced changes in isotope fluxes and voltage-clamp holding current, respectively.